1. Field of the Invention
The present invention relates generally to an apparatus and method in a screening process for printing patterns on electronic components and other substrates, and, more particularly, to vacuum assisted screening greensheet substrates containing vias and both vias and large openings which greensheets are used to make a cavity multilayer ceramic package.
2. Description of Related Art
The use of a mask, template or stencil to control the pattern for material deposition on a structure is well known. Some structures are electronic components such as multilayer ceramic components and the disclosure will be directed to such components for convenience. The term "mask" will be used herein to refer to a mask, template, stencil or other structure containing a pattern to be transferred to a substrate.
Generally, openings are provided in a sheet of material to be used as a mask in the pattern which corresponds to the desired pattern to be imparted onto a structure such as a greensheet. Such a mask is placed against the structure surface and deposition of a patterning material on the selected portions of the surface through the mask openings are made. Typical electronic component structures upon which material depositions are made are integrated circuit wafers, greensheets and substrates which are used to mount thereupon various kinds of electronic devices.
The fabrication of multilayer ceramic semiconductor packages is well known and is generally made by stacking and bonding together flexible paper-like sheets commonly referred to as greensheets. Greensheet segments of desired size and configuration are punched to provide via holes which interconnect the various package layers and, by a screen printing technique, a conductive paste fills the via holes and a via connecting conductive circuit pattern is applied to the face of the greensheet as required. Such greensheets, after screening, are assembled in a stack, pressed and subsequently sintered in an oven at a relatively high temperature.
Critical to the fabrication process is the screening operation since the ceramic greensheet or other substrates are typically relatively fragile and the design must be closely controlled because of the low pattern tolerances for these type electronic components.
There are a variety of masks which may be used for screen printing and an emulsion-treated wire mesh screen is commonly used with and without metal foils to screen print vias and conductive patterns on the surfaces of ceramic and other substrates in the process of manufacturing electrical circuit devices. In the typical screen printing process, the screen is treated with an emulsion to render it impervious to the through-flow of the printing substance. The emulsion is dried and thereafter removed in pre-selected areas which correspond to the pattern to be printed. The design is typically formed using photolithographic techniques as is well known in the art. The screen having the pattern thereon is placed over the printing surface of the substrate and a printing substance is deposited onto the screen frame. An automatic or semi-automatic screen printing system is actuated to move a flexible squeegee across the screen to push the printing substance through the areas of the screen which have been made pervious by emulsion removal.
A typical printing screen comprises a network or mesh of fine wires which have been treated with an emulsion to render the screen impervious to the fabrication substance. A typical screen is made of crossing wires of about 0.7 mil to 4.5 mil thickness. A metal foil is typically bonded to the screen to give the screen greater accuracy for small and/or delicate patterns and to provide a "snap-off" effect for cleanly removing the screen from the substrate after the squeegee passes over the screen.
When the greensheet or other electronic component substrate has vias, especially vias having a high via height to via diameter ratio, a vacuum chuck is usually required to both hold the greensheet in place and provide a vacuum to "pull" metal paste into the vias for a complete via fill. Vacuum assisted via-fill, however, can result in the patterning material passing through the via and surrounding the back side of the via. Further, when screening substrates that contain via openings and also large openings such as to make a cavity package, the mesh screen gets sucked down into the cavity opening of the greensheet during the screening operation because of the vacuum. This bowing of the screen causes the mask when lifted off the greensheet, to abruptly separate at the cavity openings resulting in splattering and possibly damaging the mask and the coated circuit pattern on the substrate surface. Blocking the vacuum from the large openings of the electronic component substrate has not been found to be effective because of the lateral migration of the vacuum through the greensheet and the vacuum blocking device.
A number of patents have issued in this area including U.S. Pat. Nos. 3,486,441; 3,769,908; 4,084,506; 4,784,310; 4,803,110; 4,902,371; 5,003,870 and 5,249,520. The disclosures of all the above patents are incorporated herein by reference.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a vacuum assisted apparatus for screen printing a greensheet or other electronic component substrate having vias and substrates and greensheets having both an open area and vias.
A further object of the invention is to provide a method for vacuum assisted screen printing a greensheet or other electronic component substrate having vias and substrates and greensheets having both an open area and vias.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.